I wish that a fuel-truck, assigned to the red-circled depot, could only refuel planes parked in the red area; the same with a fuel-truck assigned to the blue-circled lot with planes parked in the blue area… but unluckily the two trucks often choose to refuel planes at the other side of the airport stretching turn-around times and generating delays.

Yeah, it’s not always we have time to reply to every post or question unfortunately. Service vehicles consider distance just the same as employees do, or at least should, so if you have available vehicles in that depot and other ones further away get the job assigned then that does indeed sound like a bug.